Local Microflow Control Using Photothermal Viscosity Distribution

A novel method of microflow control by locally heated liquid using an optical technique is described in this paper. Microflow control in the present study utilizes temperature dependence of the fluid property which becomes dominant in microscale. Since it is known that viscosity has strong temperature dependence, a local viscosity distribution has a potential to change microflow behavior. The purpose of the present study is to validate this concept of microflow control by using the local distribution of viscosity. In order to induce the temperature variation, photothermal effect is utilized. Absorption of a laser beam causes the local heating spot. This method has attractive features such as non-intrusive and high time- and spatial- resolution, and also has a possibility for a flexible flow control. We have developed an experimental system to irradiate focused laser beam on a flow in a microfluidic device and to measure velocity and temperature field of the microflow. The local temperature rise in microchannel flow is generated by a focused laser spot. During the laser irradiation, the velocity profile of the liquid flow in microchannel with 500 μm in width and 50 μm in height was measured by micro-PIV (Particle Image Velocimetry). The temperature measurement of the flow field was performed by micro-LIF (Laser-Induced Fluorescence). Around the heated area, the local increase of the flow velocity can be observed. It is found that the effect depends on the laser intensity and is independent of the bulk velocity. In addition, numerical simulation was conducted to identify the dominant factor causing the velocity change. As a simulation result, the cause of the velocity variation is the viscosity decrease corresponding to the temperature rise in fluid. Possibility of microflow control using the photothermal viscosity distribution is confirmed.